He added: “If you are standing right next to the bomb when it goes off and the explosion does not kill you there will be some toxic material in the air for a bit, but the radiation is not going to cause you that much of a problem.”

Far more dangerous, he said, would be something like cesium-137, which comes in powder form and dissolves in water.

He did say that he was surprised the university was allowed to keep the uranium, which he described as “a big amount,” after the war. He also said the ambassador, Mohamed Ali Alhakim, should have gone to the U.N. nuclear agency, not the secretary-general.

“It’s clear what the ambassador is trying to get out of this — help to deal with ISIS,” he said.

ISIS militants have swept across swaths of Iraq and are threatening to fracture the country. ISIS, short for Islamic State of Iraq and al-Sham, aims to establish an Islamic state across the broader Middle East.

“It would just be a case of getting the fire department to wash down the pavements,” one expert says.

The U.N. nuclear agency itself said Thursday it believed that whatever nuclear material has fallen under ISIS control was “low-grade” and did not pose a high risk.

“Nevertheless, any loss of regulatory control over nuclear and other radioactive materials is a cause for concern,” said Gill Tudor, a spokeswoman for the agency. The agency plans to seek further details, she said.

Senior U.S. officials also told NBC News that the uranium was not enriched, and thus could not be turned into something of counterterrorism concern.

Kelly, who retired from the nuclear agency in 2005, stressed his experience in the field and said he “respects danger,” but he said that if ISIS “comes to my neighborhood and blows up a dirty bomb with uranium, we would deal with it.”

“Let’s just wash down the pavements and that would be it,” he said.

Asked whether Iraq would be as equipped to handle such an event, he said: “It would just be a case of getting the fire department to wash down the pavements, so, yes, I think they could just about manage.”

Andrea Mitchell and Erin McClam of NBC News contributed to this report. Reuters also contributed.